Dielectric liquids



Patented Dec. 13, 1938 PATENT OFFICE 2.139.904 mnuao'rarc uoums Arthur A. Levine and Oliver W. Cass, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemonrs & ration of Delaware Company, Wilmington, Del., a corpo- No Drawing; Application December 5, 1935, Serial No. 53,099

13 Claims.

, This invention relates to insulating fluids particularly adapted for use in electrical equipment. In electrical apparatus such as transformers, switches, electric bushings, cables, fuses and similar equipment, it is usual to provide an insulating medium which is ordinarily termed the dielectric liquid. This invention is concerned with certain novel and improved dielectric liquids which possess superior electrical properties, are noninflammable and do not produce inflammable gases whensubjected to decomposition.

A dielectric or insulating liquid in some kinds of electrical equipment such as transformers, serves a dual function. Not only does it act as an insulating medium and so prevent the trans-= fer of electricity through the dielectric and out of the electrical apparatus, but it also serves to take up and convey to cooling surfaces heat generated within the apparatus. These may be especially provided cooling means or may consist merely of the external surface of the housing in which the apparatus is positioned, which housing is exposed to the external atmosphere. In other electrical apparatus, such as in insulated current carrying cables, the dielectric liquid serves essentially as a non-conductor and not as a heat transfer medium.

In the past it has been customary to use petroleum products, such as the mineral oils, as

dielectric liquids in electrical equipment. These oils, comprising ordinary paraffinic or naphthenic hydrocarbons or mixtures of these types of hydrocarbons have not proven generally satisfactory. Although possessing certain advantages by reason of their availability and relatively low cost the mineral oils also possess certain very serious disadvantages. These disadvantages, in

general, render the use of electrical equipment containing petroleum insulating liquids somewhat hazardous.

One disadvantage of the mineral oils is that they tend to form deposits, generally known as sludge, during the period of their use. This is due to oxidation and decomposition and also resuits to some extent in the formation of acids which is equally undesirable. The sludging of dielectrics during use is a serious disadvantage and requires either replacement of the oil or treatment to remove the sludge and revivify the dielectric. A satisfactory dielectric liquid should not sludge or undergo substantial deleterious change during use and it is one of the objects of this invention to provide a liquid which does not possess this undesirable disadvantage characteristic of the mineral oils.

The electrical properties of the mineral oils are fairly satisfactory, their power factors being relatively low. However, the dielectric constant of the mineral oils is not as high as might be desired when the oils are used as dielectrics in capacitors. This is not particularly detrimental, however, as a highdielectric constant is not an advantage when the oil is employed as the dielectric liquid in transformers. Generally the power factor of a dielectric is an index of the 10 amount of power dissipated in heat during the operation of a transformer in which the dielectric serves as the insulating material. While the power factor of the mineral oils is sufliciently low, a higher dielectric constant than that 1305- 16 sessed by these oils would be desirable in many instances, more especially when the oils are used in condensers as the insulating media.

But by far the most serious disadvantage of the mineral oils is the fact that they are inflammable and upon decomposition yield gases which are also inflammable and explosive when mixed with air. A fireproof insulating liquid is highly desirable since there exists the possibility of a fire occurring in the event a short circuit or similar electrical defect occurs in the electrical apparatus insulated. In the past many fires have been caused by the use of inflammable dielectric liquids. Not only is the flammability of the mineral oils a very serious deterrent to their use, but an even greater danger exists, the development of gases which are inflammable and explosive when these oils are subjected to the decom posing influence of an electric are. In the event 5 failure occurs within a piece of electrical equipment, particularly one which is operated at high voltage and large current strengths, the dielectric is subjected to an electric arc which acts to chemically break down the dielectric. In the past in many installations, particularly in transformers in which the petroleum oils have been used as dielectrics, many dangerous explosions have occurred owing to the development of gases which are'both inflammable and explosive when In the past these various disadvantages of the 55 mineral oils have been appreciated and attempts havebeen made to utilize as dielectric liquids certain non-inflammable compositions. Pre: quently halogenated, more especially, chlorinated, products have been utilized for this purpose. At present there is electrical apparatus in commercial use employing such non-ble dielectric liquids. However, many of these compositions are unsatisfactory for one reason or another, either because of their cost or because their properties, electrical or otherwise, do not render them entirely suitable for dielectric use. In order to overcome these serious defects in the dielectric compositions known to the prior art we have prepared various new dielectric compositions. These are products which consist essentially of a halogenated aryl nucleus containing an aliphatic substituent, of which the ethylchlorobenzenes are typical compounds. We have found that chlorinatedbenzenes containingv an ethyl substituent, are especially suitable for dielectric use when there are present three, four or five atoms of chlorine per molecule of ethyl benzene. Our invention consists essentially in using as dielectric liquids in electrical equipment various mixtures of ethyltrichlorobenzene, ethyltetrachlorobenzene and ethylpentachlorobenzene, the constituents of these mixtures being so selected as to yield a dielectric suitable for the particular use to which it is to be applied.

There are certain essential requirements which a commercially satisfactory dielectric liquid must meet. The liquid must be stable and must not sludge or require frequent reconditioning. Since many pieces of electrical equipment are subjected to relative extremes of temperature it is desirable that the dielectric liquid be not too viscous at the lower temperatures and essential that it have a relatively high boiling point and not volatilize at or near the higher temperatures to which the electrical equipment may be subjected Moreover, it must have a relatively low "freeze point. This means that the temperature at which solid crystals will begin to deposit in the dielectric liquid must be one which is not too high. While the freeze point or point at which crystals deposit may be somewhat higher than the lowest temperature to which the electrical apparatus may be subjected during use it is essential that complete solidification shall not occur at any temperature encountered during use. Ordinarily since heat is developed during operation of the electrical device in which the dielectric functions as the insulating medium the temperature of the dielectric will be generally considerably higher than the outside temperature and for this reason the freeze point does not necessarily have to be as low as the lowest temperature encountered in the climatic region wherein the electrical equipment is used if there is little likelihood of the apparatus standing idle for extended intervals.

The electrical properties of the dielectric liquid must render it suitable for commercial use as the insulating liquid in electrical equipment. This means that the ohmic resistance must be relatively high and that the power factor should be low enough to fall within the range generally regarded as acceptable for dielectric liquids. The properties enumerated are, of course, in addition to what may be termed the chemical requirements, i. e., stability so that the material remains substantially unchanged during use and does not attack metals or any other material tion of benzene which has previously been ethylated. A process for preparing such compounds by first ethylating benzene and then chlorlnating the product until there are present from about 3.5 to 5.0 atoms of chlorine for each molecule of ethyl benzene is described in our copending application, Serial Number 68,919, filed March 14, i936. Preferably ethylbenzene is chlorinated until from 4.0 to 4.8 atoms of chlorine per molecule of ethylbenzene have been introduced. The product produced in this way is principally a mixture of ethyltetrachlorobenzene and ethyl- L pentachlorobenzene.

There are, of course, three isomeric ethyltetrachlorobenzenes and four isomeric ethyltrichlorobenzenes. The products prepared in accordance with our invention probably include all of the isomers of the tetraand ethyltrichlorobenzenes in varying amounts. There is, of course, only one ethylpentachlorobenzene.

The mixture prepared by the direct chlorination of ethylbenzene until there are present from 4.0 to 4.8 atoms of chlorine per molecule of ethylbenzene, in accordance with a preferred embodiment of the method described in the previously referred to co-pending patent application, may be utilized directly as a dielectric liquid in electrical equipment; This product consists substantially entirely of 'a mixture of all three isomers of ethyltetrachlorobenzene with ethylpentachlorobenzene. Other products such as ethyltrichlorobenzene are present in but extremely small amounts. Its boiling point falls substantially within the range 260 C. to 305 C. Upon cooling, solid crystals will ordinarily deposit approximately within the range of temperatures, 5 to 10 C. When there are present 4.0 atoms of chlorine per molecule of ethylbenzene the freeze point is below 0 C.; when there are 4.8 atoms of chlorine present the freeze point is approximately 35 C. Throughout this specification the temperature at which crystals will deposit upon prolonged cooling is termed the freeze point.

The dielectric constant at room temperature of this mixture of the ethyltetrachlorobenzene isomers and ethylpentachlorobenzene is approximately 4.8. This constant drops slightly as the temperature is raised. The power factor of such a mixture has been found to be very low, not ex-- ceeding about 2% measured at 80 C. after a test period of 3 days. The dielectric strength is ordinarily within the range 40 to 45 kilovolts, which is greater than the dielectric strength of the usual transformer oils. Such oils usually have dielectric strengths within the range 30 to 35 kilovolts.

The mixture prepared by chlorination until the chlorine content ranges from 4.0 to 4.8 atoms of chlorine per molecule of ethylbenzene may also be split by fractional distillation into several frac tions. Typical portions having boiling points to, operates under conditions so that chlorination of the side chain is avoided.

Pure ethylpentachlorobenzene is a solid having a melting point of 58 C. and a boiling point of approximately 305 C. We have used this product in admixture with suitable diluents which function to lower the freeze point of the resulting dielectric liquid.' It may also be used alone where its solid characteristics are not objectionable as in condensers. and in insulating various other pieces of electrical equipment. Its electrical properties are essentially identical with those given for the various fractions or mixtures thereof in table III. V

If desired, the freeze point of any of the dielectric liquids prepared as described may be lowered by the addition of any suitable and well known diluent used for a similar purpose in dielectric liquids now known to the art. Thus, for example, trlchlorobenzene prepared by the direct chlorination of benzene may be employed. The addition of such a diluentis frequently desirable to decrease the viscosity without affecting the electrical properties to such an extent as to render the liquid no longer suitable for dielectric use.

It is also possible to vary the flash points of our novel dielectric compositions by the addition of suitable diluents. Frequently such diluents" are also added for the purpose of lowering the what in composition without departing from the scope of our invention. We do not desire that the various numerical proportions and properties of the dielectric liquids and mixtures herein disclosed be construed as restrictive, except as necessitated by the scope of the appended claims.

We claim:

l. A method of preparing a dielectric material for use in electrical apparatus which comprises the steps of chlorinating ethylbenzene until the chlorine content ranges from 3.5 to 5.0 atoms per molecule of ethylbenzene, separating the product into fractions having boiling points falling within the ranges 270 to 285 C. and 285 to 305 C., and admixing these two fractions in suitable proportions so that the final product is a liquid having satisfactory electrical properties and a viscosity and freeze point rendering said liquid satisfactory for use as a dielectric medium.

2. A process for preparing an insulating composition ior use in electrical apparatus which comprises the steps of chlorinating ethylbenzene until the chlorine content ranges from 3.5 to 5.0 atoms per molecule of ethylbenzene, separating the resulting product into fractions having boiltrical properties rendering it suitable for use as a liquid dielectric.

3. A method of preparing a dielectric material suitable for use as an insulating medium in electrical apparatus which comprises the steps of chlorinating ethylbenzene until the chlorine content ranges from 3.5 to 5.0 atoms of chlorine per molecule of ethylbenzene, separating the product into fractions having boiling points falling within the ranges 260 to 270 C., 270 to 285 C. and 285 to 305 C., and admixing the two first fractions in various proportions so that the resulting product is a liquid having physical and electrical properties rendering it suitable for use as an insulating medium in electrical apparatus.

4. The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material which comprises nucleus chlorinated ethylbenzene containing at least 3 atoms of chlorine per molecule of ethylbenzene in amounts sufficient to impart dielectric properties thereto.

5. The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material comprising a liquid which consists princpally of ethyltetrachlorobenzene and ethylpentachlorobenzene.

6. The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material which comprises, in amounts sufllcient to impart dielectric properties thereto, ethyltetrachlorobenzene and ethyltrichlorobenzene.

7. The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material comprising ethylpentachlorobenzene and ethyltrichlorobenzene.

8. The method of insulating elements in electrical apparatus which comprises interposing therebetween a dielectric material comprising ethyltrichlorobenzene, ethyltetrachlorobenzene, and ethylpentachlorobenzenc, said compounds being present in amounts suificient to impart dielectric properties to the composition.

9. The method of insulating elements in, electrical equipment which comprises interposing therebetween .a dielectric material comprising ethyltrichlorobenzene in amounts suflicient to impart dielectric properties thereto.

10- The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material comprising ethyltetrachlorobenzene in amounts suflicient to impart dielectric properties thereto.

11. The method of insulating elements in electrical equipment which comprises interposing therebetween a dielectric material which contains ethylpentachlorobenzene in amounts sumcient to impart dielectric properties thereto.

12. .The method of insulating elements in electrical apparatus which comprises interposing therebetween a dielectric medium which comprises, in amounts suiiicient to impart dielectric properties thereto, a mixture of chlorinated ethyl benzene having a boiling point falling within the range 270 to 305 C. and containing, as a diluent, a mixture of chlorinated ethylbenzene having a i2); (i,ling point falling within the range 260 to 13. The method of insulating a conductor from a casing which comprises interposing a dielectric material between the conductor and the, casing, said dielectric material comprising ethyltetrachlorbenzene and ethylpentachlorbenzene.

ARTHUR A. LEVINE. OLIVER W. CASS.

within certain definite ranges following table:

are given in the By suitably regulating the amount of chlorine introduced within the range indicated, 4.0 to 4.8 atoms per molecule of ethylbenzene, the relative amounts of each of these fractions may be varied til within relatively wide limits. The relative amounts of each of these fractions are illustrated in the following table:

Table II Number of chlorine atoms introduced per mole of ethylbenzene Fraction Per- Per- Per- Per- Percent cent cent cent cent Forerun 9. 3. 4 2. 4 Nil Nil I. Boiling Point 2fi0270 C 35.0 25.1 3.8 4. 9 l. 7 II. Boiling Point 270-280" C 35. 6 39. 8 34. 2 34. 3 9.4 III. Boiling Point 285-305 C., 19. 30. l 57. 9 60. 8 88. 9 Residue Nil 1.6 l. 7 Nil Nil Total 100.0 100.0 100.0 100.0 100. 0

Each one of these fractions is entirely suitable for use as a dielectric liquid and each fraction may be utilized as the dielectric in a particular piece of electrical apparatus for which its properties render it most suitable. Preferably Fraction II, or Fraction III, or mixtures of Fraction II and Fraction III are employed. Fraction II consists primarily of the three isomeric ethyltetrachlorobenzenes mixed with a smaller amount of ethylpentachlorobenzene, while Fraction III consists primarily of ethylpentachlorobenzene mixed with a smaller amount of tetrachlorethylbenzene. However, if desired, a sub fraction from the 260 to 270 C. fraction may be taken. This consists primarily of a mixture of the ethyltetrachlorobenzenes. Similarly a fraction may be separated having a boiling point falling within the range 298 to 303 C. consisting of practically pure ethylpentachlobenzene. Ethylpentachlorobenzene in substantially pure form may be separated from the fraction having boiling points within the range 285 to 305 C. by cooling and filtering off the crystalline solid, since pure ethylpentachlorobenzene is a solid at ordinary temperatures.

All of the mixtures specified are substantially noninfiammable. This applies to all mixtures prepared by chlorinating ethylbenzene in accordance with the process of our co-pending application until from 3.5 to 5.0 atoms of chlorine are present per mole of ethylbenzene. Our preferred product having from 4.0 to 4.8 atoms of chlorine per molecule of ethylbenzene and having a boiling point falling within the range 260 to 305 C. is also substantially non-inflammable even at relatively high temperatures. Not only is this product substantially non-inflammable, but of course all of the three fractions indicated above, or any of the preferred mixtures described are all substantially non-inflammable.

We have determined the flash point of the 270 to 285 C. fraction, Fraction II in Table I, and have found it to be 158 C. (316 F.) This fraction has no fire point up to 250 C. (482 F.).

Fraction III of Table I, the fraction having a boiling pointwithin the range 285-305 C., has a flash point of over 200 C. (392 F.) and no fire point in investigations carried above 275 C. (527 F). Mixtures of Fractions II and III will have a flash point between 158 C. and well over 200 C. and no fire point up to their boiling points. When subjected to the disruptive influence of an electric arc, the gaseous products obtained by dissociation when admixed with air, in all cases are entirely non-explosive. The densities of all fractions and of mixtures of these fractions, including all of the preferred fractions and mixtures described above, will fall within the density range 1.40 to 1.55, these values being determined at 30 C.

The electrical properties of all fractions may be stated to be approximately as follows:

Table III Breakdown voltage (0.1 inch gap) 40-45 kilovolts Dielectric constant at 25 C 4.8 Power factor at 25 C 0.1% Power factor at 80 C Below 3% We have found that the fraction having a boiling point within the range 260 to 270 C., indicated in Table I as Fraction I, is very useful for lowering the freeze point or viscosity of mixtures including one or more of the remaining two fractions. By admixing various proportions. of this fraction with the other fractions, products may be prepared which have viscosities and freeze points falling within any desired ranges.

As previously described our invention also con templates the use in dielectric mixtures or as a dielectric liquid of that product prepared by chlorinating ethylbenzene until from 3.5 to 4.0 atoms of chlorine are present for each molecule of ethylbenzene. This product has boiling points falling within the range 235 to 285 C. and consists of the ethyltetrachlorobenzene isomers together with greater or less amounts of other substances such as ethyltrichlorobenzene. When but 3.5 atoms of chlorine have been introduced the product consists of the various ethyltrichlorobenzene isomers which proportion decreases as the chlorine content is increased. While this product is less valuable for dielectric use alone, because of its less satisfactory electrical properties, it may be used in admixture with other materials. Thus its principal use is probably as a diluent for lowering the freeze point and viscosity of various dielectric mixtures. The practice of adding a diluent to adjust viscosity and freeze point so as to fall within desirable limits is discussed subsequently in this specification. However, the mixture prepared by chlorinating until there are present from 3.5 to 4.0 atoms of chlorine per molecule of ethylbenzene may also be used alone under some circumstances and in some electrical installations where high dielectric strength and low power factor are not so essential.

The density of the ethylchlorobenzene mixtures having a chlorine content falling within the ratios specified will generally range from 1.35 to 1.40 at 30 C. The flash point is ordinarily above 140 C.

When more than 4.8 atoms of chlorine are present per molecule of ethylbenzene the product resulting 'from the chlorination is substantially 

